Boiler for use with gaseous fuel or oil

ABSTRACT

The boiler has two horizontal rows of steel tubes with steel fins staggered with respect to each other and extending between tube sheets. Headers are associated with the tube sheets to provide multiple passes. The tube sheets are in sections having joints between the sections so that the tube sheets are articulated to allow freedom of axial expansion relatively of tubes in respective passes to prevent failure at tube ends. The tubes are copper lined and the faces of tubes sheets within headers are copper clad with copper inserts joining ends of copper tube linings with copper claddings.

[ Apr. 23, 1974 United States Patent 1191 Block et al.

[ 1 BOILER FOR USE WITH GASEOUS FUEL 3,134,366 5/1964 Miller OR OIL 3,380,516 4/1968 Kaye 3,534,712 10/1970 Reynolds... 122/367 X [75] Inventors: Leo Block, Woodland Hills; Larry J. Ashton, Thousand Oaks, both of Calif.

Primary Examiner-Kenneth W. Sprague Attorney, Agent, or Firm--Herzig & Walsh ABSTRACT [73] Assignee: Raypak, lnc., Westlake Village,

Calif.

[22] 1 Filed:

May 1972 The boiler has two horizontal rows of steel tubes with Appl. No.: 258,444 steel fins staggered with respect to each other and extending between tube sheets. Headers are associated with the tube sheets to provide multi ple passes. The tube sheets are in sections having joints between the sections so that the tube sheets are articulated to allow freedom of axial expansion relatively of tubes in re- [58] Field of Search...

spective passes to prevent failure at tube ends. The

tubes are copper lined and the faces of tubes sheets within headers are copper clad with co pper inserts ds of copper tube linings with copper cladn e g .m .m w S T N m M e m C e D E P N U m B 122/358 M dings.

122/4 165/180 18 Claims, 11 Drawing Figures 6/1901 Hubbard....................... 123,612 2/1872 Buckingham 1,589,646 6/1926 Hicks............... 2,828,723 4/1958 Mlller PATENTEU APR 2 3 m4 SHEET 1 BF 3 1 BOILER FOR USE WITH GASEOUS FUEL OR OIL SUMMARY OF THE INVENTION The invention is a type of boiler adapted for use of gaseous fuel and also oil. It incorporates finned tubes which are unconventional in the oil fired boiler industry. Copper tubes tend to react chemically with the sulfur in the fuel and oil and, therefore, have not been used in oil fired boilers. The objectives of the invention and the improved construction whereby the objectives are realized are described in detail hereinafter.

BACKGROUND OF THE INVENTION The conventional single pass water tube boiler used for oil firing is a straight tube boiler, the tubes being made of steel. Because of the many tubes, the boiler requires a significant amount of stored water within the tubes, and this has a definite operating disadvantage.

Because of the many tubes, the water flows through the tubes at a relatively low velocity, and this tends to increase scale formation and reduces the rate of heat transfer.

The major disadvantage with a boiler of this type is the tendency for failure of the tubes at the tube sheet. This results from the different rates of expansion between the tubes.

In the unfired cold condition, the tubes are the same length, and the tube sheets are parallel.

After it has been fired for a significant period of time, the bottommost tubes attain a higher temperature than do the upper tubes. Therefore, the bottom tubes expand to a greater length than do the upper tubes. The tube sheets are no longer parallel, but have a tendency to converge out of parallelism, which produces various types of common failures known in the art.

A tube tends to expand, and the tube sheet tends to remain stationary. The tube will tend to slide in the tube sheet; and after repeated ON-OFF cycles, the tendency of this tube to slide develops into a leak. The water flows from the header into the space between the tube and tube sheets and into the combustion product area.

In another type of failure, the tube sheet is forced out of parallel and actually causes a deformation of the tube. There is, in effect, a cold work of the tube metal which eventually may result in a crack through the wall of the tube.

In a conventional multi-pass Water tube boiler, the water enters into. the inlet of a header which includes a partition. The water then flows through the first group of tubes, normally referred to as the first pass, and into the opposite header. In this header, the water makes a U turn and flows through the second pass tubes and then back to the original header. The water then again makes a U turn, flows through the third pass tubes into the opposite header and then flows out of the boiler. As the water is constantly increasing in temperature as it goes from one pass to the other, the third pass tubes attain a greater temperature and, therefore, a greater length than the second pass tubes. In a like manner, the second pass tubes attain a greater temperature and a greater length than the first pass tubes. In the multi-pass boilers, the same problem exists as in the single pass boilerin that the tube sheets are forced out of parallel. The tube ends tend to move relative to the tube sheet, and the ends of the tube tend to be cold worked and develop cracks.

The main disadvantage of the conventional prior art water tube boiler include the following:

1. A greater number of tubes must be used to attain proper heat transfer surface. Thus, the boiler stores a relatively large amount of water. When the boiler goes off because the thermostat is satisfied, all of the heat stored in the water and in the metal of the steel tubes is eventually lost to the atmosphere. This results in a loss of operating economy. When the boiler is again fired, by action of the thermostat, it is necessary to again put a significant amount of heat into the water and into the mass of metal tubes before the boiler can be brouhgt up to the proper operating temperature. Thus, the less water in a boiler and the less mass of metal in the heat exchanger, the greater the operating economy of the boiler.

2. Even a conventional multi-pass boiler has so may tubes required to attain the necessary heat transfer area, that the water velocity through the tubes is of an extremely low magnitude. The herein inventors have discovered from use of copper fin tube boilers that scale formation can be minimized and even prevented if water velocities can be kept at a relatively high level. It is preferred to maintain a water velocity of 8 to 10 feet per second if the water hardness is between 0 to 15 grains per gallon. When the water hardness increases to greater than 16 grains per gallon, it is recommended that velocity of 10 to l3 feet per second be used. These velocities are virtually impossible in the conventional multi-pass boiler because of the greater number of tubes required. Thus, these boilers have a marked tendency to form scale; whereas the steel fin tube boilers of the herein invention minimize such a tendency.

3. Due to unequal expansion of tubes, the tubes tend to expand relative to the tube sheet and also to be cold worked by the tube sheet causing tube leaks.

Among the objects of the herein invention, are to eliminate or minimize the three major undesirable features of the conventional water tube boiler. Steel finned tubes are employed in the heat exchanger. The use of fins increases the heat transfer area of each tube; therefore, fewer tubes are required. The use of fewer tubes results in a smaller volume of water stored within the boiler and also a smaller mass of metal. Thus, when the boiler is shut off, a smaller amount of heat'is lost to the atmosphere, and the boiler more rapidly achieves operating temperature after firing. In this manner, the main disadvantage of the conventional tube boiler is circumvented.

The use of finned tubes, which also permits the use of fewer tubes, allows greater velocity flow through the tubes. As previously described, this tends to reduce to an absolute minimum scale formation on the tubes and increased heat transfer so less tubes are required/In order to still attain higher velocities (on the order of 5 to 8 feet per second), the heat exchanger is circuited in four horizontal passes. In this manner, the herein boiler effects a significant improvement over the second objection of a conventional water tube boiler.

Particular constructional improvements are embodied in the boiler to realize the objective of overcoming the aforesaid disadvantages or deficiencies. In a preferred form. of the invention, there are only two horizontal rows of steel tubes with steel fins. The rows are staggered with respect to each other so that each row is substantially exposed to radiant heat. Tube sheets and headers are arranged to provide for multiple passes through the boiler. The tube sheets are constructed in sections with joints between sections so that the tube sheets at each end are articulated to provide for freedom of relative axial expansion of tubes to fully prevent failure at the ends of the tubes. The tube sheet sections are suspended by suspension members which allow relative movement of tube sheet sections at both ends in a direction axially of the tubes.

Further improvements reside in that the tubes are copper lined and the faces of tube sheets within headers are copper clad or covered. Further, the ends of the copper liners are joined or connected with the tube sheet covers by copper inserts so that fluid within the boiler is only exposed to the copper. Further objects in addition to those set forth above reside in the provision of the particular tube arrangements in the boiler; the particular construction of the tube sheets to prevent tube failures; and the full copper lining within the interior of the boiler.

Tube replacement has been a severe problem in the art. A further object of the invention is to provide a novel means and technique for tube replacement which is adapted particularly to the boiler construction having two-horizontal rows of tubes.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and additional advantages of the invention will become apparent from the following detailed description and annexed drawings, wherein:

FIG. 1 is a partly broken away schematic perspective view of a preferred form of the invention;

FIG. 2 is an exploded perspective view illustrating the articulated tube sheet and header construction;

FIG. 3 is an end view of the exemplary form of the invention;

FIG. 4 is a sectional view taken along the line 44 of FIG. 3;

FIG. 5 is a sectional view taken along the line 55 of FIG. 3;

FIG. 6 is a partial sectional view of a tube end in a tube sheet illustrating copper lining and cladding;

FIG. 7 is a view similar to FIG. 6 after expanding or flaring the end of the tube and liner;

FIG. 8 is a schematic view illustrating the articulated tube sheet, headers, and multi-pass construction;

FIG. 9 is a detail view illustrating a joint between tube sheet sections;

FIG. 10 is a schematic view illustrating replacement of a tube in the upper row of the boiler; and

FIG. 11 is a schematic view illustrating replacement of a tube in the lower row of the boiler.

DESCRIPTION OF THE PREFERRED EMBODIMENT Flg. 1 is a broken away perspective view of the boiler of this invention. At the left is a burner 10 capable of operating on fuel oil and gaseous fuel. A refractory type combustion chamber 12 having refractory l3 similar to that used in copper finned boilers insures that substantially all of the heat flows vertically through the heat exchanger 14 and into the flue outlet 16. The heat exchanger consists of two horizontal rows of tubes. The two rows are staggered to attain maximum exposure to radiant heat and optimum contact with the flue products.

FIGS. 1 through 4 show the heat exchanger tubes located in two horizontal rows and in staggered orientation. FIG 8 shows a four-pass arrangement of tubes as will be described more in detail presently. Numerals 20 and 22 designate a tube in each row. The tubes are steel with steel fins extending between tube sheets 28 and 28.

The use of more than two horizontal rows would tend to force the tube sheets out of parallel. For this reason as explained in the foregoing, the heat exchanger is limited to two horizontal rows of tubes. All tubes are exposed to substantially the same amount of radiation and flue product temperature.

The tube sheets 28 and 28 are made in segments joined or articulated together. If this were not so, the tubes would be forced out of parallel (along the horizontal axis), as the water flowing through the last pass is significantly hotter than the water flowing through the first pass. Thus, the tubes of the last pass attain a higher temperature that the tubes of the previous passes. Here again, the unequal expansion of tubes would create a problem if it were not for the segmented tube sheets.

FIG. 8 is a schematic plan view of the four pass heat exchanger employing the use of segmented tube sheets. The segments or tube sheet sections are designated 34-38. The first pass tubes 42 attain the length of L, after the boiler attains the normal operating temperature. The water enters inlet header 44 and then flows through the opposite return heater 46 into the second pass tubes 48 and is again heated. These tubes attain the length of L These tubes are free to expand to their new length because tube sheet section 35 is separate and not mechanically (not rigidly) attached to tube sheet section 34. The water then flows through another return header 52 and enters the third pass tubes 54. In a like manner, the third pass tubes attain the length of L and are free to expand because tube sheet section 38 is not mechanically attached to tube sheet section 37. The third pass water then enters still another return header 56 and flows through the fourth pass tubes 58. These tubes attain the length of L They are free to attain their expanded length without being put into constraint. Tube sheet section 36 is not mechanically attached to tube sheet section 35. the third pass tubes 58 connect to outlet header 64. In this manner, the boiler eliminates the third and most important deficiency of the conventional boiler.

The segmented or vertebrae tube sheet sections permit free expansion of the tubes to any length to which they may expand without:

1. Putting the tubes in constraint;

2. Without creating a condition in which the tubes tend to move relative to the tube sheet sections and thereby developing a leak; and

3. Without forcing the tube sheets out of parallel which causes cold working and ultimate failure of the tube ends, as described in the foregoing.

The problem in designing segmented tube sheet sections is preventing the leakage of flue products between the individual tube sheet segments. The tube sheet sections have grooves or recesses milled into the vertical edges thereof as shown at and 72 in FIG. 9. A stainless steel link 74 is positioned in the grooves. The link is substantially narrower than the width of the milled grooves or slots. The tube sheet segments are free to move relative to one another causing the link to oscillate within the two grooves or slots. The link prevents the leakage of flue products between the tube sheet sections, the vertical edges of the tube sheet sections being juxtaposed as shown with the slots opposed to each other and link 74 extending or bridging between them.

An alternate method of obtaining this sealing arrangement may be utilized. Silicone rubber or some other resilent material capable of withstanding the temperature, maybe used instead of the link 74 being of stainless steel to bridge the gap between the tube sheet segments. Because of the resilience of the material, the tube sheet segments are free to move relative to one another; and the resilient material seals and prevents leakage of flue products.

The tube sheet sections must also be in positioned so that they do not move in a vertical direction relative to each other. They are supported in the combustion chamber in a manner to accommodate this purpose. FIGS. 2 through 5 show the preferred detailed construction of the tube sheet sections, headers, and the manner of support of the sheets in the combustion chamber. As will be observed in FIG. 8, there are two headers at one end of the boiler and three at the other end where the inlet and outlet headers are positioned. Otherwise, the construction at opposite ends of the tube passes are alikelThe headers have a configuration as illustrated in FIG. 2. Inlet header 44 has an inlet boss 80, and outlet header 64 has an outlet boss 82. On the top and bottom of the headers are a series of mounting lugs. Those at the top are designated at 83, 84, and 85. Those at the bottom are designated by similar refer ence characters primed. (See FIG. 3). Those at the opposite ends are designated by the same reference characters, including the letter a. The headers are secured to the tube sheets by means of tie bolts which engage the lugs 83-85 and which thread into threaded holes in the tube sheets as may be seen in FIGS. 4 and 5. In the sectional views, FIGS. 4 and 5, the tie bolts are designated at 90, 90, 90a, 900'. At the outer end of the tie bolt 90 are spaced nuts 92 and 94 with a spacer sleeve 96 captured between them. Numeral 100 designates a support or suspension member, and numeral 102 designates a second suspension member. The lower end of suspension member 100 has apertures 104 and 106 as may be seen in FIG. 5. Tie bolt 90 and sleeve 96 are in aperture 104 and a corresponding tie bolt and sleeve are in aperture 106. All of the tie bolt assemblies are alike, so only one need be described in detail. The upper end of suspension member 100 is secured to frame member 110 which is part of the steel frame which supports the refractory material constituting the combustion chamber by bolt 112. In the exemplary structure shown, two of the suspension members are provided at one end and one at the other end designated at 101. They are all alike. Suspension member 100 bridges between and supports headers 44 and 52, and suspension member 102 bridges between and supports headers 52 and 64. There is an assembly at the opposite end which corresponds to members 100 and 102.

Headers 44 and 52 are held together at the bottom by straps 113 and 115. Headers 52 and 64 are held together at the bottom by corresponding straps.

Referring to tie bolt 90, sleeve 96 spaces nuts 92 and 94. This construction is the same for all of the support members, such that the headers and tube sheetsat both ends can freely expand in a direction axially of the tubes in the manner illustrated and described in connection with FIG. 8.

Referring to header 52, it is secured to tube sheet 35 with gasket 116 between the header and tube sheet for sealing. Numeral 120 designates copper cladding or covering on the face of tube sheet 35 within header 52. This construction is similar within all headers. The copper covering is directly on the surface of the tube sheet, and the gasket is of a type such that it is between only the contacting surfaces of the header and tube sheet.

Copper/Bronze Waterways The boiler of the invention embodies a construction such as to shield parts not made of copper from expo sure to liquid in the boiler. This construction is an improvement in the construction shown in US. Pat. No. 3,207,215 of Alfred Whittell, Jr. FIGS. 6 and 7 are partial sectional views of the joint between the end of the tube and the tube sheets, such as tube 20 and tube sheet section 35. As previously pointed out, the tubes are made of steel with steel fins as designated at 126. In tube 20 is a copper sleeve 128, the end of which extends beyond the tube itself. Tube sheet section 35 has a bore to receive the tube, and it has a counterbore 132 as shown. The copper cladding or covering on the tube sheet section is shown at 120, and it has an inturned circular flange 134 which extends back into counterbore 132 as shown. Numeral 138 designates a copper insert ring having an angular cross section as shown such that it has an in-turned flange part 140 which abuts against steel tube 20. This flange has a bore 142 into which the end of copper sleeve 128 fits. Insert ring 138 extends into counterbore 132 with flange 134 trapped between the insert and wall of counterbore 132. FIG. 6 shows the construction of the joint before final flaring of the end of the tube. FIG. 7 shows the final construction in which the end of steel tube 20 and lining 128 are flared outwardly at an angle as shown so that insert ring 138 is deformed. Flange 140 is angled outward slightly as shown with the deformation as shown at 144 at the corner of the insert ring. This construction provides a firm joint with the parts firmly juxtaposed together so that the parts not made of copper are shielded from the liquid in the boiler. It is understood that the headers themselves are preferably of bronze so that complete copper/bronze waterways are realized without any other surfaces being exposed to the liquid in the boiler.

The lining is expanded tightly within the steel tubes without any air gap to provide good heat transfer. The expanded end of the steel tube locks to the tube sheet and provides a leak-proof joint.

In an alternative form of construction, ring 138 is omitted and a flexible nonmetallic ring is used which might be an O-ring positioned in counterbore 132 between flange 134 and the end of tube 20.

Tube Replacement The problem of replacing finned tubes in the field is a severe one. Probably for this reason, finned tubes have not been used in boilers where multiple vertical tube rows are required.

The replacement of a conventional straight tube is rather simple. The tube to be replaced is cut, usually with a cutting torch, between the tube sheets. The section of tubing remaining in the tube sheet is collapsed with a chisel and then forced out of the tube sheet hole. Frequently on multi-tube boilers, it is necessary to remove undamaged tubes in order to obtain access to the tube which needs replacement. The replacement tube is then fed through one tube sheet and into the other tube sheet. The tube is then expanded at each end with a conventional tube roller.

The removal of finned tubes would follow the same procedure as straight tubes; however, the finned tubes cannot, obviously, be passed through the hole in the tube sheet because of the large diameter fins. An arrangement has been evolved for replacement of tubes in the field which employs the use of a special field replacement tube. The construction and technique are illustrated in FIGS. 10 and 11. Regular tubes are designated at 20 and 22 and numeral 21 designates a special field replacement tube. Numeral 13 designates the refractory of the combustion chamber. This field replacement tube has several fins removed from one end 23. This end of tube is swaged into a smaller diameter which permits the insertion of the undersized tube end in one hole of the tube sheet in a position as seen in FIGS. 10 and 11. The tube is forced into this hole until the opposite end of the finned tube clears the opposite tube sheets. The tube is then lowered to a horizontal position, and the standard end of the tube is inserted into the tube sheet. Both tube ends are then expanded to make a tight joint with the tube sheet. No problem is encountered in expanding the swaged down end into a standard size tube sheet hole.

The tube with the swaged down end is inserted as shown and is then lowered to a horizontal position (in FIG. 10), and the opposite end of the tube is inserted in the tube sheet. The standard end of the tube sheet must be rolled first. This acts to prevent rotation of the tube when the swaged down end is expanded into the tube sheet hole.

FIG. 10 shows replacement of a top boiler tube. This requires the removal of the two covers 15 and 16 over access openings. FIG. 11 shows the replacement of a bottom boiler tube. In order to place the tube in position as shown, it is necessary to obtain clearance between the bottom of the tube sheets 28 and 28' and the refractory 13. To accomplish this, the upper covers over the access openings are first removed and lifting angles 17 and 17a are bolted to the tube sheets using the header attaching studs. Tension bolts 19 and 19a are used to lift the tube sheets vertically to obtain the necessary clearance for insertion of the tube 21 as illustrated.

The foregoing disclosure is representative of a preferred form of the invention and is to be interpreted in an illustrative rather than a limiting sense, the invention to be accorded the full scope of the claims appended hereto.

What is claimed is:

1. In a boiler construction, in combination: means comprising a plurality of steel tubes having steel fins; and tube sheet means having the tubes extending between them, there being only two horizontal rows of tubes, the tubes in the respective rows being staggered with respect to each other so that the tubes in one row do not directly overlie tubes in another row so that tubes are exposed equally to radiant heat.

2. A boiler construction as in claim 1, wherein the tubes are arranged so as to provide a plurality of passes so related to each other that fluid traverses the passes in sequence.

3. In a boiler construction, in combination: means comprising a plurality of finned tubes; and tube sheet means having the tubes extending between them, there being only two horizontal rows of tubes, the tubes in the respective rows being staggered with respect to each other so that the tubes in one row do not directly overlie tubes in another row so that tubes are exposed equally to radiant heat, the tubes being arranged so as to provide a plurality of passes so related to each other that fluid traverses the passes in sequence, said tube sheet means comprising a plurality of tube sheet sections at each end of the tubes, means providing joints between tube sheet sections allowing relative movement between adjacent tube sheet sections in the direction of the axis of the tube whereby to allow for variable expansion of tubes in different passes.

4. A boiler construction as in claim 3, wherein said tube sheet sections are so interrelated to each other that the tube means in each pass have freedom to expand without disturbing the joints between the ends of tubes and the tube sheets.

5. A boiler construction as in claim 3, including header means associated with tube sheet sections, said tube sheet sections and header means including tube sheet sections with an associated header which provides communication between the tube means of one pass and tube means of another pass with the tube means of the one pass and of the other pass being free to expand axially relatively to each other.

6. A boiler construction as in claim 3, adjacent tube sheet sections having portions juxtaposed against each other, said juxtaposed portions having recesses therein opposed to each other, and a sealing means disposed in the opposed recesses for allowing limited relative movement as between the juxtaposed portions.

7. A boiler construction as in claim 6, wherein the sealing means is made of flexible material forming a seal.

8. A boiler construction as in claim 1, including supporting means for said tube sheets, said supporting means having attachment to said tube sheets whereby to allow relative movement of the tube sheets in the direction of the axes of the tubes.

9. A boiler construction as in claim 8, wherein said supporting means are in the form of members suspending the tube sheets from a frame structure.

10. In a boiler construction, in combination: means comprising tube sheets having a plurality of metal tubes extending therebetween, the ends of said tubes extending into openings in the tube sheets; means comprising copper linings within the tubes, said tube sheets having a surface forming a part of a header, said surface having a copper cladding thereon; and means whereby the ends of the copper lining are joined to said copper cladding so that fluid is exposed only to the copper.

11. A boiler construction as in claim 10, including means comprising a copper ring at the end of a tube positioned to be interposed between the end of a copper tube lining and the copper cladding on said surface of the tube sheet.

12. A boiler construction as in claim 11, wherein each of said openings in said tube sheet comprises a bore and a coun'terbore, said copper tube lining extending beyond the end of the tube at the counterbore, said copper cladding having a circular flange part extending against an extending end part of the copper lining and i an extending part in said counterbore juxtaposed against said circular flange on the copper cladding.

providing joints between tube sheet sections allowing relative movement between adjacent tube sheet sections in the direction of the axes of the tubes whereby to allow for variable expansion of the tubes.

l A boiler construction as in claim 14, wherein said tube sheet sections are so interrelated to each other that tubes connected to different tube sheet sections have feeedom to expand.

16. A boiler construction as in claim 15 including tube sheet sections with an associated header which provide communication betwe the tube means of one pass through the boiler and tube means of another pass through the boiler with the tube means of the different passes being free to expand axially relative to each other.

17. A boiler construction as in claim 15, wherein said tube sheet sections have edge portions juxtaposed against each other, said portions having recesses therein opposed to each other, and a holding means disposed in the opposed recesses for allowing limited relative movement between the juxtaposed portions.

18. A boiler construction as in claim 14, including means between adjacent tube sheet sections for sealing the escape of flue products. 

1. In a boiler construction, in combination: means comprising a plurality of steel tubes having steel fins; and tube sheet means having the tubes extending between them, there being only two horizontal rows of tubes, the tubes in the respective rows being staggered with respect to each other so that the tubes in one row do not directly overlie tubes in another row so that tubes are exposed equally to radiant heat.
 2. A boiler construction as in claim 1, wherein the tubes are arranged so as to provide a plurality of passes so related to each other that fluid traverses the passes in sequence.
 3. In a boiler construction, in combination: means comprising a plurality of finned tubes; and tube sheet means having the tubes extending between them, there being only two horizontal rows of tubes, the tubes in the respective rows being staggered with respect to each other so that the tubes in one row do not directly overlie tubes in another row so that tubes are exposed equally to radiant heat, the tubes being arranged so as to provide a plurality of passes so related to each other that fluid traverses the passes in sequence, said tube sheet means comprising a plurality of tube sheet sections at each end of the tubes, means providing joints between tube sheet sections allowing relative movement between adjacent tube sheet sections in the direction of the axis of the tube whereby to allow for variable expansion of tubes in different passes.
 4. A boiler construction as in claim 3, wherein said tube sheet sections are so interrelated to each other that the tube means in each pass have freedom to expand without disturbing the joints between the ends of tubes and the tube sheets.
 5. A boiler construction as in claim 3, including header means associated with tube sheet sections, said tube sheet sections and header means including tube sheet sections with an associated header which provides communication between the tube means of one pass and tube means of another pass with the tube means of the one pass and of the other pass being free to expand axially relatively to each other.
 6. A boiler construction as in claim 3, adjacent tube sheet sections having portions juxtaposed against each other, said juxtaposed portions having recesses therein opposed to each other, and a sealing means disposed in the opposed recesses for allowing limited relative movement as between the juxtaposed portions.
 7. A boiler construction as in claim 6, wherein the sealing means is made of flexible material forming a seal.
 8. A boiler construction as in claim 1, including supporting means for said tube sheets, said supporting means having attachment to said tube sheets whereby to allow relative movement of the tube sheets in the direction of the axes of the tubes.
 9. A boiler construction as in claim 8, wherein said supporting means are in the form of members suspending the tube sheets from a frame structure.
 10. In a boiler construction, in combination: means comprising tube sheets having a plurality of metal tubes extending therebetween, the ends of said tubes extending into openings in the tube sheets; means comprising copper linings within the tubes, said tube sheets having a surface forming a part of a header, said surface having a copper cladding thereon; and means whereby the ends of the copper lining are joined to said copper cladding so that fluid is exposed only to the copper.
 11. A boiler construction as in claim 10, including means comprising a copper ring at the end of a tube positioned to be interposed between the end of a copper tube lining and the copper cladding on said surface of the tube sheet.
 12. A boiler construction as in claim 11, wherein each of said openings in said tube sheet comprises a bore and a counterbore, said copper tube lining extending beyond the end of the tube at the counterbore, said copper cladding having a circular flange part extending into the counterbore; and means comprising a copper ring having a cross section to provide a part engaging against an extending end part of the copper lining and an extending part in said counterbore juxtaposed against said circular flange on the copper cladding.
 13. A boiler construction as in claim 12, wherein the end of each tube and its lining is flared outwardly into said counterbore.
 14. In a boiler construction, in combination: means comprising tube sheet means having tubes extending therebetween, the ends of the tubes being sealed to the tube sheet means; tube sheet means comprising a plurality of tube sheet sections at each end of the tubes; means providing joints between tube sheet sections allowing relative movement between adjacent tube sheet sections in the direction of the axes of the tubes whereby to allow for variable expansion of the tubes.
 15. A boiler construction as in claim 14, wherein said tube sheet sections are so interrelated to each other that tubes connected to different tube sheet sections have feeedom to expand.
 16. A boiler construction as in claim 15, including header means associated with tube sheet sections, including tube sheet sections with an associated header which provide communication betwe the tube means of one pass through the boiler and tube means of another pass through the boiler with the tube means of the different passes being free to expand axially relative to each other.
 17. A boiler construction as in claim 15, wherein said tube sheet sections have edge portions juxtaposed against each other, said portions having recesses therein opposed to each other, and a holding means disposed in the opposed recesses for allowing limited relative movement between the juxtaposed portions. 